Of light emitting diodes (hereinafter, referred to conveniently as “LEDs”) which emit light when a forward current is passed through the pn junction of compound semiconductors, LEDs of AlGaAs and GaAs families are widely used in the fields of infrared communications and remote control communications. For example, as LEDs for use in infrared communications which require higher power and increased speed of operation to meet with larger volume communications, those of double-hetero (DH) structure are used. In an LED of DH structure, an active layer is more advantageous for high power and rapid responsiveness if it is thinner. And, generally it is made to have a thickness of 1 μm.
Patent Reference 1 below describes an LED of DH structure as mentioned above in which the active layer has an effective impurity (dopant) added thereto and formed of germanium (Ge) and has a thickness of 0.5 to 1.5 μm so that the LED is obtained that is high in power and reliability and satisfactory in responsiveness.
Also, as LEDs for use in remote control communications, those of homojunction type are used which are by liquid phase growth of GaAs. The pn junction in such an LED is formed utilizing the so-called pn reversal, or the inversion from p type to n type, of silicon (Si) used as a dopant in the liquid phase, at a growth temperature. Since Si forms a deeper impurity level than other possible dopants in GaAs, the GaAs homojunction type LED has emission wavelengths widely distributed in a wavelength range longer than the GaAs absorption edge (870 nm). However, a long diffusion length of its carriers leaves its pulse responsiveness slow. As a result, its rise time (Tr) and fall time (Tf) have had their limits of several μ seconds.    Patent Reference 1: Japanese Patent laid open application: JP H08-293622 A